Network Working Group W. Kumari Internet-Draft Google Obsoletes: 7710 (if approved) E. Kline Intended status: Standards Track Loon Expires: October 1, 2020 March 30, 2020 Captive-Portal Identification in DHCP / RA draft-ietf-capport-rfc7710bis-03 Abstract In many environments offering short-term or temporary Internet access (such as coffee shops), it is common to start new connections in a captive portal mode. This highly restricts what the customer can do until the customer has authenticated. This document describes a DHCP option (and a Router Advertisement (RA) extension) to inform clients that they are behind some sort of captive-portal enforcement device, and that they will need to authenticate to get Internet access. It is not a full solution to address all of the issues that clients may have with captive portals; it is designed to be used in larger solutions. The method of authenticating to, and interacting with the captive portal is out of scope of this document. RFC7710 used DHCP code point 160. Due to a conflict, this document specifies TBD. [ This document is being collaborated on in Github at: https://github.com/capport-wg/7710bis. The most recent version of the document, open issues, etc should all be available here. The authors (gratefully) accept pull requests. Text in square brackets will be removed before publication. ] Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any Kumari & Kline Expires October 1, 2020 [Page 1] Internet-Draft DHCP Captive-Portal March 2020 time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 1, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. The Captive-Portal Option . . . . . . . . . . . . . . . . . . 3 2.1. IPv4 DHCP Option . . . . . . . . . . . . . . . . . . . . 4 2.2. IPv6 DHCP Option . . . . . . . . . . . . . . . . . . . . 5 2.3. The Captive-Portal IPv6 RA Option . . . . . . . . . . . . 5 3. Precedence of API URIs . . . . . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 4.1. IETF params Registration . . . . . . . . . . . . . . . . 6 4.1.1. Registry name: Captive Portal Unrestricted Identifier 6 4.2. BOOTP Vendor Extensions and DHCP Options Code Change . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 7. Normative References . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 10 Appendix B. Changes from RFC 7710 . . . . . . . . . . . . . . . 10 Appendix C. Observations From IETF 106 Network Experiment . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction In many environments, users need to connect to a captive-portal device and agree to an Acceptable Use Policy (AUP) and / or provide billing information before they can access the Internet. Regardless of how that mechanism operates, this document provides functionality Kumari & Kline Expires October 1, 2020 [Page 2] Internet-Draft DHCP Captive-Portal March 2020 to allow the client to know when it is behind a captive portal and how to contact it. In order to present users with the payment or AUP pages, presently a captive-portal enforcement device has to intercept the user's connections and redirect the user to a captive portal server, using methods that are very similar to man-in-the-middle (MITM) attacks. As increasing focus is placed on security, and end nodes adopt a more secure stance, these interception techniques will become less effective and/or more intrusive. This document describes a DHCP ([RFC2131]) option (Captive-Portal) and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that informs clients that they are behind a captive-portal enforcement device and how to contact an API for more information. 1.1. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. The Captive-Portal Option The Captive Portal DHCP / RA Option informs the client that it may be behind a captive portal and provides the URI to access an API as defined by [draft-ietf-capport-api]. This is primarily intended to improve the user experience by showing the user the captive portal information faster and more reliably. Note that, for the foreseeable future, captive portals will still need to implement the interception techniques to serve legacy clients, and clients will need to perform probing to detect captive portals. Clients that support the Captive Portal DHCP option SHOULD include the option in the Parameter Request List in DHCPREQUEST messages. DHCP servers MAY send the Captive Portal option without any explicit request. In order to support multiple "classes" of clients (e.g. IPv4 only, IPv6 only with DHCPv6 ([RFC3315]), and IPv6 only with RA) the captive network can provision the client with the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The captive portal operator SHOULD ensure that the URIs provisioned by each method are equivalent to reduce the chance of operational problems. The maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, so URIs longer than 255 bytes should not be provisioned via IPv6 DHCP or IPv6 RA either. Kumari & Kline Expires October 1, 2020 [Page 3] Internet-Draft DHCP Captive-Portal March 2020 In all variants of this option, the URI MUST be that of the captive portal API endpoint, conforming to the recommendations for such URIs [draft-ietf-capport-api]. A captive portal server MAY redirect requests that do not have an Accept header field ([RFC7231] Section 5.3) containing a field item whose content-type is "application/capport+json" to the URL conveyed in the "user-portal-url" API key. When performing such content negotiation ([RFC7231] Section 3.4), captive portals implementors need to keep in mind that such responses might be cached, and therefore SHOULD include an appropriate Vary header field ([RFC7231] Section 7.1.4) or mark them explicitly uncacheable (for example, using Cache-Control: no-store [RFC7234] Section 5.2.2.3). A captive portal MAY do content negotiation ([RFC7231] section 3.4) and attempt to redirect clients querying without an explicit indication of support for the captive portal API content type (i.e. without application/capport+json listed explicitly anywhere within an Accept header vis. [RFC7231] section 5.3). In so doing, the captive portal SHOULD redirect the client to the value associated with the "user-portal-url" API key. The URI SHOULD NOT contain an IP address literal. Networks with no captive portals MAY explicitly indicate this condition by using this option with the IANA-assigned URI for this purpose (see Section 4.1.1). Clients observing the URI value "urn:ietf:params:capport-unrestricted" MAY forego time-consuming forms of captive portal detection. 2.1. IPv4 DHCP Option The format of the IPv4 Captive-Portal DHCP option is shown below. Code Len Data +------+------+------+------+------+-- --+-----+ | code | len | URI ... | +------+------+------+------+------+-- --+-----+ o Code: The Captive-Portal DHCPv4 Option (TBD) (one octet) o Len: The length, in octets of the URI. o URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]). Note that the URI parameter is not null terminated. Kumari & Kline Expires October 1, 2020 [Page 4] Internet-Draft DHCP Captive-Portal March 2020 2.2. IPv6 DHCP Option The format of the IPv6 Captive-Portal DHCP option is shown below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | option-code | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . URI (variable length) . | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o option-code: The Captive-Portal DHCPv6Option (103) (two octets) o option-len: The length, in octets of the URI. o URI: The URI for the captive portal API endpoint to which the user should connect (encoded following the rules in [RFC3986]). See [RFC7227], Section 5.7 for more examples of DHCP Options with URIs. Note that the URI parameter is not null terminated. 2.3. The Captive-Portal IPv6 RA Option This section describes the Captive-Portal Router Advertisement option. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | URI . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Captive-Portal RA Option Format Type 37 Length 8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 bytes. URI The URI for the captive portal API endpoint to which the user should connect. This MUST be padded with NULL (0x00) to make the Kumari & Kline Expires October 1, 2020 [Page 5] Internet-Draft DHCP Captive-Portal March 2020 total option length (including the Type and Length fields) a multiple of 8 bytes. Note that the URI parameter is not guaranteed to be null terminated. 3. Precedence of API URIs A device may learn about Captive Portal API URIs through more than one of (or indeed all of) the above options. It is a network configuration error if the learned URIs are not all identical. However, if the URIs learned are not in fact all identical the captive device MUST prioritize URIs learned from network provisioning or configuration mechanisms before all other URIs. Specifically, URIs learned via any of the options in Section 2 should take precedence over any URI learned via some other mechanism, such as a redirect. If the URIs learned via more than one option described in Section 2 are not all identical, this condition should be logged for the device owner or administrator. Implementations can select their own precedence order. 4. IANA Considerations This document requests one new IETF URN protocol parameter ([RFC3553]) entry. This document also requests a reallocation of DHCPv4 option codes (see Appendix C for background). Thanks IANA! 4.1. IETF params Registration 4.1.1. Registry name: Captive Portal Unrestricted Identifier Registry name: Captive Portal Unrestricted Identifier URN: urn:ietf:params:capport-unrestricted Specification: RFC TBD (this document) Repository: RFC TBD (this document) Index value: Only one value is defined (see URN above). No hierarchy is defined and therefore no sub-namespace registrations are possible. Kumari & Kline Expires October 1, 2020 [Page 6] Internet-Draft DHCP Captive-Portal March 2020 4.2. BOOTP Vendor Extensions and DHCP Options Code Change [ RFC Ed: Please remove before publication: RFC7710 uses DHCP Code 160 -- unfortunately, it was discovered that this option code is already widely used by Polycom (see appendix). Option 114 (URL) is currently assigned to Apple (RFC3679, Section 3.2.3 - Contact: Dieter Siegmund, dieter@apple.com - Reason to recover: Never published in an RFC) Tommy Pauly (Apple) and Dieter Siegmund confirm that this codepoint hasn't been used, and Apple is willing to relinquish it for use in CAPPORT. Please see thread: https://mailarchive.ietf.org/arch/msg/captive-portals/ TmqQz6Ma_fznD3XbhwkH9m2dB28 for more background. ] The IANA is requested to update the "BOOTP Vendor Extensions and DHCP Options" registry (https://www.iana.org/assignments/bootp-dhcp- parameters/bootp-dhcp-parameters.xhtml) as follows. Tag: 114 Name: DHCP Captive-Portal Data Length: N Meaning: DHCP Captive-Portal Reference: [THIS-RFC] Tag: 160 Name: REMOVED/Unassigned Data Length: Meaning: Reference: [RFC7710][Deprecated] 5. Security Considerations An attacker with the ability to inject DHCP messages or RAs could include an option from this document to force users to contact an address of his choosing. As an attacker with this capability could simply list himself as the default gateway (and so intercept all the victim's traffic); this does not provide them with significantly more capabilities, but because this document removes the need for interception, the attacker may have an easier time performing the attack. As the operating systems and application that make use of this information know that they are connecting to a captive-portal device (as opposed to intercepted connections) they can render the page in a sandboxed environment and take other precautions, such as clearly labeling the page as untrusted. The means of sandboxing and user interface presenting this information is not covered in this document - by its nature it is implementation specific and best left to the application and user interface designers. Kumari & Kline Expires October 1, 2020 [Page 7] Internet-Draft DHCP Captive-Portal March 2020 Devices and systems that automatically connect to an open network could potentially be tracked using the techniques described in this document (forcing the user to continually authenticate, or exposing their browser fingerprint). However, similar tracking can already be performed with the presently common captive portal mechanisms, so this technique does not give the attackers more capabilities. Captive portals are increasingly hijacking TLS connections to force browsers to talk to the portal. Providing the portal's URI via a DHCP or RA option is a cleaner technique, and reduces user expectations of being hijacked - this may improve security by making users more reluctant to accept TLS hijacking, which can be performed from beyond the network associated with the captive portal. By simplifying the interaction with the captive portal systems, and doing away with the need for interception, we think that users will be less likely to disable useful security safeguards like DNSSEC validation, VPNs, etc. In addition, because the system knows that it is behind a captive portal, it can know not to send cookies, credentials, etc. By handing out a URI which is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious. 6. Acknowledgements This document is a -bis of RFC7710. Thanks to all of the original authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve Sheng), and original contributors. Also thanks to the CAPPORT WG for all of the discussion and improvements including contributions and review from Joe Clarke, Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens Schimpe, Martin Thompson, Michael Richardson, Remi Nguyen Van, Bernie Volz, and Tommy Pauly. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, . Kumari & Kline Expires October 1, 2020 [Page 8] Internet-Draft DHCP Captive-Portal March 2020 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, . [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2003, . [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and S. Krishnan, "Guidelines for Creating New DHCPv6 Options", BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, . [RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng, "Captive-Portal Identification Using DHCP or Router Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710, December 2015, . 7.2. URIs [1] https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments [2] https://tickets.meeting.ietf.org/wiki/CAPPORT [3] https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP- Standardization-160-vs-66/td-p/72577 Kumari & Kline Expires October 1, 2020 [Page 9] Internet-Draft DHCP Captive-Portal March 2020 Appendix A. Changes / Author Notes. [RFC Editor: Please remove this section before publication ] From initial to -00. o Import of RFC7710. From -00 to -01. o Remove link-relation text. o Clarify option should be in DHCPREQUEST parameter list. o Uppercase some SHOULDs. Appendix B. Changes from RFC 7710 This document incorporates the following changes from [RFC7710]. 1. Clarify that IP string literals are NOT RECOMMENDED. 2. Clarify that the option URI SHOULD be that of the captive portal API endpoint. 3. Clarify that captive portals MAY do content negotiation. 4. Added text about Captive Portal API URI precedence in the event of a network configuration error. 5. Added urn:ietf:params:capport-unrestricted URN. 6. Notes that the DHCP Code changed from 160 to 114. Appendix C. Observations From IETF 106 Network Experiment During IETF 106 in Singapore an experiment [1] enabling Captive Portal API compatible clients to discover a venue-info-url (see experiment description [2] for more detail) revealed that some Polycom devices on the same network made use of DHCPv4 option code 160 for other purposes [3]. The presence of DHCPv4 Option code 160 holding a value indicating the Captive Portal API URL caused these devices to not function as desired. For this reason, this document requests IANA deprecate option code 160 and reallocate different value to be used for the Captive Portal API URL. Kumari & Kline Expires October 1, 2020 [Page 10] Internet-Draft DHCP Captive-Portal March 2020 Authors' Addresses Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: warren@kumari.net Erik Kline Loon 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: ek@loon.com Kumari & Kline Expires October 1, 2020 [Page 11]